Fluorescent material for self-erasable writing, authentic security labeling, currency counterfeit prevention and processes for the preparation thereof

ABSTRACT

The present invention provides a novel fluorescent molecule having formula 1 which exhibit emission color change when in contact with moisture useful for the preparation of fluorescent paper, allowing self-erasable writing, security label for document authenticity, check and prevention of currency counterfeit. Compound of formula 1 when coated on paper results in blue emitting surfaces on which writing is possible using water as ink. The images undergo self-erasal after 6 hours making the paper reusable or instantly with hot air. The images are visible only on illumination with a UV lamp having wavelength 365 nm. The material can be coated on any document to create security label which changes color on touching with a wet surface or water pen and will go back to the native color when the moisture is dried off after a time frame. This strategy is used to create a security label for currency bills for checking original and fake currency notes or any other documents for authenticity. The technique is also useful for temporary writing on recyclable papers.

CLAIM OF PRIORITY

This application is a divisional of U.S. application Ser. No.13/571,763, filed 10 Aug. 2012 now U.S. Pat. No. 9,012,688, which claimsthe benefit of priority of Indian Patent Application Serial Number 2294DEL/2011, filed on 12 Aug. 2011, which applications are incorporated byreference as if reproduced herein and made a part hereof in theirentirety, and the benefit of priority of each of which is claimedherein.

FIELD OF THE INVENTION

The present invention relates to fluorescent material useful forself-erasable writing, authentic security labeling, currency counterfeitprevention and processes for the preparation thereof.

More particularly, the present invention relates to the process for thepreparation of a fluorescent molecule having general formula 1 which canbe used for hand written self-erasable images using water as ink whichhas application in currency counterfeit prevention, detection and alert.The present invention also relates to a process for the preparation offluorescent papers for writing and preparation of security labels forchecking authenticity of important documents such as visa.

wherein

BACKGROUND OF THE INVENTION

Reference may be made to moisture sensitive materials based onfluorescent molecules wherein this type of materials can be used tosense moisture by monitoring their optical changes. J. Kunzelman, B. R.Crenshaw, C. Weder, J. Mater, Chem. 2007, 17, 2989. The drawbacks arefluorescence of the film is quenched and the process is irreversible.

Reference may be made to U.S. Pat. No. 3,618,765 wherein currencycounterfeit detector is used to detect the chromamorphic response andmagnetic characteristics of US paper currency. The drawback of thisinvention is the counterfeit also incorporates the chromamorphicresponse and magnetic characteristics.

Reference may be made to U.S. Pat. No. 5,662,735 wherein chemicalsolution containing iodine is used to detect currency counterfeit. Thedrawback of this invention is the counterfeit also incorporates thisproperty of chemical solution of iodine and thereby making it difficultto differentiate from the genuine currency.

Reference may be made to U.S. Pat. No. 5,757,001 wherein detectingcurrency counterfeit by measuring the reflectance of near infrared beamsand comparing the reflectance values with those from genuine currency.The drawback of this invention is the property of reflectance of nearinfrared beams can be incorporated to fake currency making it difficultto differentiate from genuine currency.

Reference may be made to self-erasable and rewritable materials in whichinformation is written over metastable nanoparticle “inks” wherein thesematerials can be useful for storing sensitive or temporary informationare known from the work of R. Klajn, P. J. Wesson, K. J. M. Bishop, andB. A. Grzybowski, Angew. Chem. Int. Ed. 2009, 48, 7035 and US patent20100328410A1. The drawbacks are written images are visible in normalday light and light of certain wave length is used to write the images.

Reference may be made to U.S. Pat. No. 7,315,876 filed by XeroxCorporation wherein self-erasable printing paper allows recycling ofpaper by using a polymer incorporated photochromic molecule for printingon paper. The drawbacks of this invention are written images are visibleto eyes under normal light and images are printed on paper using lightof certain wave length.

Reference may be made to rewritable photonic paper with hygroscopic saltsolution as ink was made by J. Ge, J. Goebl, L. He, Z. Lu, and Y. YinAdv. Mater., 2009, 21, 4259. The drawbacks of this invention are thewriting made by the hygroscopic salt solution as ink is notself-erasable.

Reference may be made to rewritable phosphorescent paper by the controlof competing kinetic and thermodynamic self-assembling events was madeby A. Kishimura, T. Yamashita, K. Yamaguchi and T. Aida Naturematerials, vol 4, July 2005. The drawbacks of this invention arewritings made at particular temperature and erasable using differentrate of temperature.

Reference may be made to mechanochromic fluorescent materials as acandidate for writing and imaging by G. Zhang, J. Lu, M. Sabat and C. L.Fraser, J. Am. Chem. Soc. 2010, 132, 2160 and S. J. Yoon, J. W. Chung,J. Gierschner, K. S. Kim, M. G. Choi, D. Kim and S. Y. Park, J. Am.Chem. Soc. 2010, 132, 13675. The drawbacks of this invention arewritings made by applying force and reusability can be done usingthermal annealing or exposing to solvent vapors.

Reference may be made to Oligo (p-phenylenevinylene) derivatives thatare known to form self-assembled structures in polystyrene matrix whichchange the fluorescence from green to blue upon heating and hence isuseful for thermal imaging by S. Srinivasan, P. A. Babu, S. Mahesh, andA. Ajayaghosh J. Am. Chem. Soc. 2009, 131, 15122. The drawbacks arewritings are made using thermal pen which is not friendly to use andwritten images are not self-erasable.

However as evident from above references self-erasable hand writingusing water as an ink is unknown, the fluorescent compound havingformula 1 that changes fluorescence color when in contact with water isnot known, use of formula 1 for security application is not known.

OBJECTIVES OF THE INVENTION

The main object of the present invention is to provide a fluorescentmaterial for self-erasable writing, authentic security labeling,currency counterfeit prevention and processes for the preparationthereof which obviates the drawbacks as detailed above.

Another objective of the present invention is to provide a process forthe preparation of fluorescent molecule having formula 1.

Yet another objective of the present invention is to use water as an inkfor writing.

Yet another objective of the present invention is to prepare arewritable paper in which the written information is invisible undernormal light and can be read only under light with 365 nm wavelength.

Yet another objective of the present invention is to provide afluorescent security label for documents.

Still another objective of the present invention is to detect, preventand alert currency counterfeit by fluorescence color change in contactwith water.

BRIEF DESCRIPTION OF THE DRAWINGS

The patent or application file contains at least one drawing executed incolor. Copies of this patent or patent application publication withcolor drawing(s) will be provided by the Office upon request and paymentof the necessary fee.

FIG. 1 represents flow chart illustrating the entire processes involvedin this work.

FIG. 2A Shows the emission spectra of OPE formula 1 in monomer state(THF) and aggregated state (THF-Water mixture) at 1×10⁻⁵ Mconcentration. Excitation wavelength 340 nm. FIG. 2B Shows thephotograph of OPE formula 1 in monomer state (left) and aggregated state(right) under UV light.

FIG. 3 shows emission spectra of thin film a) before and b) afterspraying water above it. Excitation wavelength is 340 nm.

FIG. 4 shows optical characterization of thin film: Photograph of thinfilm with blue (left) and green (right) fluorescence under UV light.

FIG. 5 shows transmission electron microscopic (TEM) image of formula 1in THF-Water (1:9) mixture at 5×10⁻⁵ M concentration.

FIG. 6 shows dynamic light scattering analysis of spherical aggregateswith different size in THF-Water mixture.

FIG. 7 shows the rewritable nature of fluorescent paper under UV light.

FIG. 8 shows the self-erasable images with time under UV light.

FIG. 9 shows the photograph of Indian currency under UV light havingwave length 365 nm with letter “NIIST” written using chloroform solutionof formula.

FIG. 10 shows the photograph of Indian currency under UV light (365 nm)after writing the letters “NIIST” and strike across by water pen.

FIG. 11 shows the photograph of Indian currency under UV light (365 nm)coated with chloroform solution of formula 1 and above tick mark doneusing water pen.

FIG. 12 shows Scheme 1, which represents process steps for thepreparation of compound of formula 1.

SUMMARY OF THE INVENTION

Accordingly, present invention provides compound of general formula 1

-   -   wherein

In an embodiment of the present invention, said compound is useful asfluorescent material for self-erasable writing, authentic securitylabeling, currency counterfeit prevention.

In another embodiment, present invention provides a process for thepreparation of compound of general formula 1 comprising the steps of:

-   -   i. refluxing compound 3 with compound 2 in tetrahydrofuran and        triethylamine in the ratio ranging between 1:1 to 1:1.5 in the        presence of catalyst at temperature in the range of 55 to 65° C.        for period in the range of 11 to 13 h under argon atmosphere;

-   -   ii. cooling the reaction mixture as obtained in step (i) at        temperature in the range of 25 to 30° C. followed by extracting        using chloroform to obtain combined organic layer;    -   iii. washing the combined organic layer as obtained in step (ii)        with water, brine followed by drying over anhydrous sodium        sulphate and purifying using column chromatography with 10%        methanol-chloroform as eluent to obtain compound of general        formula 1.

In yet another embodiment of the present invention catalyst used in step(i) are 5 to 10 mol % Bis(triphenylphosphine)palladium(II) chloride[PdCl₂(PPh₃)₂] and 5 to 10 mol % Copper iodide (CuI).

In yet another embodiment of the present invention, yield of thecompound of general formula 1 is in the range of 72-75%.

In yet another embodiment, present invention provides a process for thepreparation of fluorescent material/paper for self-erasable writing byusing compound of formula 1 comprising the steps of:

-   -   a) dissolving 0.1 to 0.2 wt % compound of formula 1 in        chloroform under gentle heating at a temperature in the range of        35 to 45° C. to obtain a solution;    -   b) spraying the solution as obtained in step (a) over strip of a        paper and dried under vacuum to obtain fluorescent        material/paper.

In yet another embodiment of the present invention, said fluorescentpapers coated with compound having formula 1 changes the fluorescencecolor from blue to green upon spraying or touching with water over it.

In yet another embodiment of the present invention, said fluorescentpapers coated with compound having formula 1 are useful forself-erasable writing using water as ink for writing over it.

In yet another embodiment of the present invention, said fluorescentpapers coated with compound having formula 1 self-erases the temporarydata in four to six hours and abruptly when exposed to hot air.

In yet another embodiment of the present invention, temporary datawritten over the fluorescent papers coated with compound having formula1 is invisible under normal light.

DETAILED DESCRIPTION OF THE INVENTION

Present invention provides a fluorescent material for self-erasablewriting, authentic security labeling, currency counterfeit preventionand processes for the preparation thereof which comprises of synthesisof novel fluorescent molecule3,4,5-tris(2-(2-(2-methoxyethoxy)ethoxy)ethoxy)-N-(4-((4-phenylethynyl)phenyl)ethynyl)phenyl)benzamide oligo(p-phenyleneethynylene) (OPE) representedby formula 1 and coating using formula 1 for self-erasable writing,security labeling and for detection, alert and prevention of currencycounterfeit by dissolving the compound having formula 1 in chloroformand coating over the paper followed by water spray, writing with waterand applying moisture which is then exposed to ultra violet light havingwave length 365 nm which displays blue fluorescence in the absence ofwater or moisture and changes to green fluorescence in the presence ofwater and remain invisible under normal light.

FIG. 1 shows flow chart illustrating all the processes involved in thiswork. According to various exemplary embodiments the processes startfrom step S100 and continues to step 110, during which formula 1 issynthesized using compound 2 and compound 3 following scheme 1, shown inFIG. 12. Next, process continues to step S120, during which formula 1 ischaracterized using various techniques such as UV, FT-IR, ¹H NMR, ¹³CNMR and HRMS. Next, process continues to step S130, during which 3 mg offormula 1 is dissolved in 2 ml of chloroform. Next, process continues tostep S140, during which the chloroform solution of formula 1 is sprayedover paper or currency. Next, process continues to step S150, duringwhich the substrate is dried under vacuum. Next, process continues tostep S160, during which the substrate is illuminated using ultravioletlight of wavelength 365 nm. Next, process continues to step S170, duringwhich writing using water pen or touch it with moisture above thesubstrate can be made. Next, process continues to step S180, duringwhich the substrate shows green color fluorescence for moisture spreadregions. Next, process continues to step S190, during which thesubstrate is exposed to hot air or keeping at room temperature for sixhours. Next, process continues to step S200, during which the substrateshows blue fluorescence under ultraviolet light of wavelength 365 nm.Next, process finally continues to step S210, during which processesend.

The present invention provides a process for the synthesis of novelfluorescent molecule having formula 1, the said process comprising thesteps of:

Refluxing compound 3 (1 equivalent) with compound 2 (1.2 equivalents) intetrahydrofuran (dry, 10 equivalents) and triethylamine (dry, 10equivalents) in the presence of catalyst PdCl₂(PPh₃)₂ and CuI at 60° C.for 12 h under argon atmosphere. After cooling to room temperature (25°C.) the reaction mixture was extracted using chloroform. The combinedorganic layer was washed with water, brine and dried over anhydroussodium sulphate and purified using column chromatography with 10%methanol-chloroform as eluent to give formula 1 in 75% yield andcharacterized by FT-IR, UV, ¹H NMR, ¹³C NMR and HRMS.

The present invention provides a process for the preparation ofrewritable paper for temporary writing, the said process comprising thesteps of:

Preparing a solution of the molecule by dissolving 3 mg of the moleculein 2 ml of chloroform followed by gentle heating between 35° C. to 45°C. and spraying the solution obtained on a paper and dried under vacuum.The paper is blank in normal light conditions. The paper coated withmolecule having formula 1 is exposed to Ultraviolet light of wavelength365 nm. The paper showed blue fluorescence. Moisture is applied on paperand again it is exposed to Ultraviolet light of wavelength 365 nmwherein paper showed green fluorescence. The green fluorescence remainedon the paper for six hours and the fluorescence changed from green toblue instantly when the paper was exposed to hot air.

The transmission electron microscopic (TEM) image of formula 1 at 5×10⁻⁵M concentration in presence of 1:9 THF-Water mixtures showed theformation of spherical particles as shown in FIG. 5 which is theaggregation of formula 1 in water to give the fluorescence change. Thedynamic light scattering (DLS) experiment showed the spherical nature ofthe particle of formula 1 with different size as shown in FIG. 6. FIG. 2corresponds to emission changes of formula 1 with THF and THF-watermixture. The compound having formula 1 forms aggregates in the presenceof water or moisture which causes fluorescence to change from blue togreen. In the absence of water or moisture the compound having formula 1will not form aggregates and shows blue fluorescence.

Most of the self-erasable writings are visible under normal day light.In case of security reasons if one needs to hide the written informationand do not want anyone to photocopy the information, it should beinvisible to eyes in normal day light and should be visible only withthe aid of certain light with particular wavelength. Moreover theerasable recording medium should be fluorescent to prevent photocopy.The present invention satisfies all the above requirements i.e. thewritings are self-erasable, water can be used as ink for writing,writings are visible only under illumination with an ultra violet lamp,high contrast and color difference between the written and non writtenparts and instant erasal of the writing with hot air if required.

A novel fluorescent molecule having formula 1 is synthesized and themoisture responsive property of formula 1 of the current invention couldbe used in number of applications. More particularly it can be appliedin fake money prevention, detection and alert. Since the fluorescentmark made from formula 1 change its fluorescence color upon exposure tomoisture, it can be coated over the currency note. Upon spraying wateron the above currency, it changes its fluorescence color and the fakeone could not have any change. In this way the compound is highly usefulto detect, alert and prevent the circulation of fake money.

The fluorescent paper made from formula 1 can be used to store temporarydata. The secret data is written using pen containing water as ink.Written images can be self-erased within 6 hours or exposing the paperto hot air. The material described herein can be used optionally asrewritable papers were the repeated writing and rewriting is needed.

The fluorescent molecule having formula 1 can be used as a securitylabel for checking and detecting the authenticity of important documentssuch as visa by virtue of its fluorescence color change from blue togreen upon contact with moisture under UV light of wavelength 365 nm.

The self-erasable paper of the current invention uses pen containingwater as ink. This technique is environmentally friendly and low cost.

The above are illustrative of various uses/applications of theinvention. Again, the enumeration of the specific embodiments herein notto be taken as necessarily limiting on the other non-detaileduses/applications which comprise the self-erasable writing of thecurrent invention. Those of skill in art will appreciate other possibleapplication and uses of the above and methods herein.

EXAMPLES

The following examples are given by the way of illustration andtherefore should not construe to limit the scope of the invention.

Example 1 Process for the Preparation of Compound of Formula 13,4,5-tris(2-(2-(2-methoxyethoxy)ethoxy)ethoxy)-N-(4-((4-phenylethynyl)phenyl)ethynyl)phenyl)benzamideoligo(p-phenyleneethynylene) (OPE)

Synthesis of compound of formula 1 was achieved as per scheme 1, shownin FIG. 12. It was prepared by refluxing compound 3 (1 equivalent, 500mg) with compound 2 (1.2 equivalents, 150 mg) in tetrahydrofuran (dry,10 ml) and triethylamine (dry, 10 ml) in the presence of catalystPdCl₂(PPh₃)₂ (42 mg) and CuI (12 mg) at 60° C. for 12 h under argonatmosphere. After cooling to room temperature (25° C.) the reactionmixture was extracted using chloroform. The combined organic layer waswashed with water, brine and dried over anhydrous sodium sulphate andpurified using column chromatography with 10% methanol-chloroform aseluent to give compound of formula 1 in 75% yield and characterized by¹H NMR, ¹³C NMR and HRMS (High Resolution Mass Spectral).

i) FT-IR (KBr) v_(max)=1915, 2220, 2875, 3034, 3237, 3598 cm⁻¹.

ii) UV λ_(max)=335 nm.

iii) ¹H NMR (500 MHz, CDCl₃, TMS): δ=3.32 (s, 6H, OCH₃), 3.38 (s, 3H,OCH₃), 3.51-3.55 (m, 6H, OCH₂), 3.63-3.73 (m, 18H, OCH2), 3.79-3.85 (m,6H, OCH₂), 4.23-4.25 (m, 6H, OCH₂), 7.25 (s, 2H, aromatic), 7.34-7.36(m, 3H, aromatic), 7.51-7.55 (m, 8H, aromatic), 7.73 (d, 2H, aromatic),8.64 (s, 1H, NH) ppm.

iv)¹³C NMR (125 MHz, CDCl₃): δ=58.92, 69.82, 70.51, 71.88, 72.39, 88.78,89.14, 91.28, 107.98, 118.43, 120.12, 122.93, 128.38, 129.86, 131.61,132.33, 138.83, 142.02, 152.51, 165.54 ppm.

v) HRMS: calculated: C₅₀H₆₁NO₁₃, 884.02. found: 885.50.

Example 2 Preparation of Fluorescent Material

Compound of formula 1 (3 mg) was taken in clean and dry glass vial. 2 mlof chloroform was added to it and gently heated to 35° C. to make thesolution homogeneous. After cooling to 25° C. the solution was sprayedover the paper and dried in vacuum.

Example 2 Self-Erasable Writing Readable Only by Light Irradiation

Fluorescent paper prepared from compound of formula 1 can be used forwriting temporary data. The fluorescent paper is prepared by sprayingchloroform solution of compound of formula 1 and drying under vacuum.The paper showed a blue fluorescence under UV light. When a selectivepart of the paper was sprayed with water, the sprayed region aloneshowed green fluorescence while the unsprayed part retains initial bluefluorescence. This exhibits the writable nature of fluorescent paper bywater pen shown in FIG. 7.

The data written over the fluorescent paper would be self-erased within6 hours under normal condition or instantly by exposing to hot air. Thepaper was reused for repeated writing using water pen. The fluorescenceintensity remains the same after several cycles. This exhibits theself-erasable and rewritable nature of the paper for several cycles, byretaining their intrinsic property as shown in FIG. 8.

The writing is not visible under normal day light. The information inthe paper cannot be photocopied and the data will be visible only in thepresence of ultraviolet light. For protecting secured data thisinvention can be used for temporary storing of information and imageswhich will be revealed only under UV light temporarily for six hours.

Example 3 Security Label for Protecting Authenticity of Documents

Fluorescent mark made from compound of formula 1 can be used as securitylabels. Under ultraviolet light the fluorescent mark shows initiallyblue fluorescence and when water sprayed above or applied moisture thefluorescence color changes to green. The original blue fluorescence canbe attained after six hours or instantly by exposing the coated paper tohot air under ultraviolet light. This technique finds application inprotecting and detecting authenticity of important legal and secureddocuments such as visa.

Example 4 Process for the Prevention and Detection of CurrencyCounterfeit

The moisture responsive property of compound of formula 1 can be appliedin currency counterfeit detection, alert and prevention. The procedureis outlined below. A solution of compound having compound 1 was preparedby dissolving 3 mg of the compound in 2 ml of chloroform followed bygentle heating to 35° C. Then the solution was used to write the letter“NIIST” using paint brush above currency bill and dried under vacuum.Initially the letters shows blue fluorescence under UV light as shown inFIG. 9. Using a water pen a strike mark is made over the letter “NIIST”which shows green fluorescence at the marked region (FIG. 10). Theinitial blue fluorescence can be attained instantly by exposing currencyto hot air or kept at room temperature for six hours. The fluorescenceintensity remains same after repeating this process for several cycles.In another experiment to detect counterfeit money chloroform solution ofcompound having formula 1 was coated over currency and dried undervacuum. The coated surface shows blue fluorescence initially. A tickmark was made above the currency using water pen then the marked regiononly changed to green fluorescence as shown in FIG. 11.

The fluorescent mark made from compound 1 showed very goodreversibility, FIG. 2 corresponding to the fluorescence spectral changewhen repeatedly sprayed with water and dried using hot air. Thefluorescent intensity remains the same at their respective state afterseveral cycles of treatment. This exhibits the excellent reversiblenature of the fluorescence mark and retention of their inherentcharacter.

ADVANTAGES OF THE INVENTION

The main advantages of the present system include:

-   1. Synthesis of novel fluorescent molecule having formula 1.-   2. Compound having formula 1 coated over currency can be used for    the detection, prevention and alert of fake money or documents.-   3. Compound having formula 1 can be used as security labels for    important documents which showed rapid response to the moisture.-   4. The information stored in the fluorescent paper is invisible to    eyes in normal day light and will be visible only under UV light    with wave length 365 nm.-   5. Self-erasable nature of fluorescent paper allows the data to    store temporarily.-   6. Water can be used as ink for writing fluorescent paper.-   7. The fluorescent paper can be reused for several times by exposing    to hot air or kept at room temperature for few hours.-   8. Since the fluorescence color change is from blue to green, it can    be easy visualized by using UV light.-   9. Writings made above the fluorescent paper done by water pen makes    friendly to environment.

We claim:
 1. A process for the preparation of fluorescent material or paper for self-erasable writing comprising the steps of: i) dissolving 0.1 to 0.2 wt % compound of formula 1

wherein R is the group:

in chloroform to obtain a solution; ii) spraying the solution on the material or paper; and iii) drying the material or paper under vacuum to obtain fluorescent a coated material or paper.
 2. The process as claimed in claim 1, wherein the fluorescence color of the coated material or paper changes from blue to green upon spraying or contacting the coated material or paper with water.
 3. The process as claimed in claim 1, wherein water is used to write or print on the coated paper or material and the writing or printing are self-erasable.
 4. The process as claimed in claim 3, wherein the writing or printing self-erases in four to six hours and abruptly when exposed to hot air.
 5. The process as claimed in claim 3, wherein the writing or printing is invisible under visible light.
 6. Material or paper coated with a compound of the formula 1:

wherein R is the group:


7. The material or paper of claim 6, wherein the fluorescence color of the coated material or paper changes from blue to green upon spraying or contacting the coated material or paper with water.
 8. A method for self-erasable writing on a material or paper coated with a compound of the formula 1:

wherein R is the group:

the method comprising applying water in selected regions of the material or paper.
 9. The method of claim 8, wherein water is used to write or print one or more characters on the coated paper or material.
 10. The method of claim 9, wherein the one or more characters self-erase in four to six hours and abruptly when exposed to hot air.
 11. The method of claim 9, wherein the one or more characters are invisible under visible light. 